Magnesium addition alloy

ABSTRACT

AN ADDITION ALLOY FOR INTRRODUCING MAGNESIUM INTO MOLTEN CAST IRON WITH MINIMAL ACCOMPANYING SMOKE AN D FLARE CONSISTING ESSENTIALLY OF ABOUT 4% TO ABOUT 5.5% MAGNESIUM, ABOUT 0.8% TO ABOUT 1.8% CARBON, AND THE BALANCE ESSENTIALLY NICKEL, SAID ALLOY HAVING A MICROSTRUCTURE CHARACTERIZED ESSENTIALLY BY THE PRESENCE OF A EUTECTIC STRUCTURE COMPRISING A NICKEL-MAGNESIUM-CARBON COMPOUND CONTAINING ABOUT 12% MG AND 4% CARBON IN A CONTINUOUS HIGH NICKEL PHASE.

Oct. 22, 1974 P. n. RENSCHEN HAL 3,843,856 IMGNESIUH ADDITION ALLOY Filed Jan, 27, 1972 2 Sheets-Sheet 1 Oct. 22, 1974 P. D. RENSCHEN ETAL 3,843,356

MAGNESIUM ADDITION ALLOY Filed Jan. 27, 1972 2 Sheets-Sheet 2 United States Patent Ofiice 3,843,356 MAGNESIUM ADDITION ALLOY Patrick D. Renschen and Nathan L. Church, Warwick, N.Y., assignors to The International Nickel Company, Inc., New York, NY.

Filed Jan. 27, 1972, Ser. No. 221,261 Int. Cl. C22c 33/00 US. Cl. 75130 A 3 Claims ABSTRACT OF THE DISCLOSURE Directed to a magnesium-containing, nickel-base addition alloy having a materially reduced tendency to yield smoke and flare when added to molten cast iron containing no more than about 7% magnesium and about 0.5 to about 2.5% carbon.

The manufacture of ductile iron as disclosed in US. Pat. No. 2,485,760 is now well-known. The material contains a small retained amount of magnesium and contains graphite in the spheroidal form. As a result, the material has greatly improved strength and ductility as compared to common grades of gray cast iron and is widely used in many engineering applications. In producing ductile iron it is, of course, necessary that magnesium in some form be added to molten cast iron. It is known that magnesium boils at a temperature below the melting point of most gray cast iron melts. Thus, magnesium boils at about 1117 C. whereas a normal pouring temperature for molten cast iron is about 1450 C. As a result, magnesium is conventionally added to molten cast iron as an alloy with one or more metals such as nickel, silicon, iron, etc. Even so, the addition of magnesium in alloy form to molten iron is still accompanied by substantial reactivity due to the magnesium content of the alloy as evidenced by burning of the alloy on the melt surface and the evolution of magnesium oxide smoke. The amount of magnesium oxide smoke is a function of the reactivity of the alloy. Economically it is considered desirable to use magnesium addition alloys containing the maximum amount of magnesium which can be tolerated under foundry conditions. Accordingly, in most installations at which ductile iron is manufactured, the process is accompanied by greater or lesser amounts of magnesium oxide smoke within the foundry and/or the surrounding areas during the points in time at which the magnesium addition is being made to molten iron. In the normal foundry, the magnesium addition would normally be made only a few times during a 8-hour shift. Accordingly, the smoke generated by burning magnesium Will be evident for only a short period of time during the normal Working day.

With increasing emphasis upon environmental control as evidenced by more advanced governmental requirements in relation to smoke density in the working place, it is found that magnesium oxide smoke generation during ductile iron production is of such a nature as to create a violation of more advanced smoke density code requirements. It had been thought that smoke density requirements could be met through the use of dilute magnesium alloys, such as the 95% nickel-% magnesium alloy described in US. Pat. No. 2,485,760. Unfortunately, critical evaluation of the behavior of the 95 nickel-5% magnesium alloy on a foundry scale demonstrated that results tended to be erratic and that generation of magnesium oxide smoke still occurred in amounts which would be excessive under the more rigid smoke density code requirements. Accordingly, it quickly appeared that a magnesium addition allow for ductile iron was needed which would provide markedly lower smoke evolution when added to molten iron than was the case even with 3,843,356 Patented Oct. 22, 1974 dense alloys such as the nickel-5% magnesium alloy, thought on the basis of prior experience to be the best alloy available in this respect.

As a concomitant factor in relation to the development of an essentially smoke-free magnesium addition alloy, it can be pointed out that the problem of magnesium oxide smoke evolution during ductile iron production can be solved using available apparatus designed for the purpose of removing particulate solids from the atmosphere. However, the nature of the magnesium oxide smoke problem in the foundry is such that substantial over-designing of the equipment is required since the load on the dust collection apparatus would be at a high level only during the time the magnesium oxide smoke was being generated, but at a relatively low level during other times of the day. This required over-designing would lead to uneconomically high costs in relation to the installation of conventional dust collection equipment for the purpose of controlling the magnesium oxide smoke problem in the foundry. This cost problem is particularly severe in the case of existing foundries, especially smaller ones.

We have now discovered a new magnesium-containing alloy which may be employed to introduce magnesium into molten cast iron with the generation of essentially no magnesium oxide smoke resulting from the use of the alloy.

It is among the objects of the present invention to provide magnesium addition alloys characterized by minimal evolution of smoke and flare when. dropped upon. the surface of a molten cast iron bath.

Other objects will become apparent from the following description taken in conjunction with the. accompanying drawing in which:

FIG. 1 is a reproduction of a photomicrograph taken at 500 diameters depicting the unetched microstructure of an alloy provided in accordance with the invention;

FIG. 2 is a reproduction of a photomicrograph taken at 500 diameters depicting the unetched microstructure of a comparable prior art alloy; and

FIGS. 3 and 4 are photographs depicting the total amount of smoke and flare generated during the addition of magnesium-containing alloys to molten cast iron under standardized conditions, with FIG. 3 depicting a condition assigned an arbitrary value of 1 and FIG. 4 depicting a condition assigned an arbitrary value of 10.

Generally speaking, the invention is directed to a nickel alloy containing about 3% to about 6.5% or 7% magnesium, at least about 0.5% to about 2.5% carbon and the balance essentially nickel. Preferably the alloys contain about 4% to about 5.5% or 6% magnesium, about 0.8% to about 1.8% carbon and the balance essentially nickel. For manufacturing purposes, a magnesium range of about 4.2% to about 4.8% and a carbon range of about 1.2% to about 1.6% is especially satisfactory. It is found that in the preferred alloys essentially no magnesium oxide smoke is produced upon addition of the solid alloy to a molten cast iron bath at a normal temperature for casting, e.g., 2650" F. The magnesium content should not exceed about 6.5% or about 7% as otherwise unacceptable amounts of smoke and flare result when the alloy is dropped upon the surface of a molten cast iron bath. It is preferred that amounts of carbon sufficient to cause precipitation of excess carbon as graphite be avoided due to the fact that free graphite in the alloy can markedly reduce the density thereof.

It is found that within the magnesium levels contemplated in accordance with the invention, the introduction of at least the minimum required amount of carbon into the alloy system drastically changes the microstructure of the resulting product. This is shown in the drawing wherein FIG. 1 is the structure of an alloy containing about 4.7% magnesium and about. 1.5% carbon with the balance essentially nickel and FIG. 2 depicts the structure of an alloy containing the same amount of magnesium, i.e., 4.7% magnesium, only 0.1% carbon and the balance essentially nickel, each photomicrograph having been taken at a magnification of 500 diameters. In the material depicted in FIG. 1, the structure is essentially a eutectic structure comprising a well-distributed mixture of a gray phase and a phase which is very high in nickel. By microprobeanalysis it has been demonstrated that the gray phase is a nickel-magnesium-carbon compound containing about 12% magnesium and about 4% carbon. The white phase is almost pure nickel. A strong disproportionation of carbon between the pro-eutectic phase and a high nickel phase has occurred. The structure depicted in FIG. 2 on the other hand consists of pro-eutectic nickel which appears white and a eutectic nickel-Ni Mg phase which appears interdendritically. The eutectic is rich in Ni Mg as predicted from the phase diagram, comprising about 70% Ni Mg phase which contains about 18% magnesium, by weight. There is approximately the same amount of pro-eutectic nickel and eutectic phases again as predicted from the phase diagram. The structure depicted in FIG. 2 indicates a large proportion of a high magnesium phase is available at the melting surface of the alloy when it is immersed in molten cast iron. On the other hand, the structure depicted in FIG. 1 is rich in a continuous, high melting point, high nickel phase which can act as a barrier to any sudden release of magnesium from the magnesium-rich areas when the alloy is immersed in molten cast iron. It is thought that the marked difference in microstructure between the alloy of the invention as depicted in FIG. 1 and the known 95% nickel-5% magnesium alloy is depicted in FIG. 2 is associated with the improvement in addition characteristics found in relation to carbon-containing alloys of the invention. It should be appreciated that more hypereutectic structures than that depicted in FIG. 1 will contain gray dendrites, while more hypoeutectic structures will contain white dendrites of the high nickel phase. In this connection, it can be pointed out that even in instances in which an alloy is sufliciently dense to sink beneath the surface of a molten cast iron bath, convection can bring to the surface liquid areas rich in magnesium due to too sudden sub-surface release of magnesium from the addition alloy and exposure of magnesium-rich molten iron to the surface can it self result in the production of magnesium oxide smoke. This factor favors treating larger and deeper baths of molten cast iron.

In order to give those skilled in the art a better appreciation of the advantages of the invention the following example is given.

EXAMPLE Two alloys within the invention were prepared which contained respectively, about 4.6% magnesium, about 2.1% carbon, and the balance essentially nickel, and about 5.2% magnesium, about 2.1% carbon and the balance essentially nickel. Companion alloys which contained only 0.1% carbon were also prepared. Standard weights of material of each alloy were prepared in cubic form. The standard shapes weighed about 50, 100 and 200 grams. Each of the thus prepared samples was added to a 30 pound bath of molten cast iron held in a ladle with a round, open, upper surface about 4 inches in diameter at temperatures of about 2550 F. and 2750 F. by dropping the sample from a fixed height of about 4 inches onto the surface of the molten cast iron bath. The smoke and flare resulting from each addition Was measured by means of a camera. Thus, the camera shutter was opened prior to the dropping of the sample upon the molten cast iron surface and remained open until all evidence of reaction between the added sample and the cast iron bath had ceased. In this way, all the smoke and flare from each addition was recorded on the camera film, thereby providing an integrated record.

On examination of the prints made as described, it was found possible to ascribe arbitrary values on a scale of 1 to 10 representing the increasing extent of smoke and flare generated in the drop tests. It was found that testing at 2550 F. in general developed lower amounts of smoke and flare than for the comparable test conducted at 2750 F. Evaluation of the photographs indicated without any question that the alloys containing carbon in accordance with the invention generated significantly less smoke and flare than did the comparable carbon-free alloys at each temperature of test and at each level of magnesium. Alloys containing 2.1% carbon and 5.2% magnesium generally resulted in the production of more smoke and flare, as reflected by the photographs, than did the alloys containing 2.1% carbon and 4.6% magnesium. Correlations made on the basis of values assigned to the photographs depicting smoke and flare were found to be statistically significant. Analysis of the data obtained led to definition of the ranges set forth hereinbefore, representing arbitrary smoke and flare values not exceeding about 4 and usually not exceeding arbitrary values of about 1 to 3.

FIG. 3 of the drawing is a photograph made in the drop test series which was assigned an arbitrary value of 1 and was determined with the alloy containing 4.6% magnesium, 2.1% carbon and a cast iron bath temperature of 2550 F. FIG. 4 of the drawing is another photograph made in the series which was assigned an arbitrary value of 10 and was determined with the alloy containing 5.2% magnesium, 0.1% carbon and a cast iron bath temperature of 2750 F., and depicts a condition outside the invention.

The molten cast iron composition employed in the test contained about 3.2% carbon, about 2.4% silicon, about 0.5% manganese, about 0.02% phosphorus, about 0.01% sulfur and the balance essentially iron. It was found during the course of the test that the magnesium recovery in the molten cast iron from the carbon-containing materials produced in accordance with the invention was, on the average, significantly better than was the magnesium recovery from the essentially carbon-free materials. This was taken as further evidence that the alloys as contemplated in accordance with the invention were quieter upon addition to the molten iron and generated less magnesium oxide smoke and flare.

The microstructure of the 4.6% magnesium, 2.1% carbon alloy was similar to that depicted in FIG. 1, although more hypereutectic, while the structure of the 4.6% magnesium, low-carbon alloy was essentially that depicted in FIG. 2. The carbon level at about 2.1% drastically changes the eutectic composition and the structure of the alloy. For the material containing about 2.1% carbon, the eutectic composition contains approximately 4% magnesium. On the other hand, the low-carbon alloy has a eutectic composition containing approximately 12% magnesium. The 2.1% carbon alloy is thus a hypereutectic alloy whereas the low-carbon alloy is hypoeutectic. In any event, the microstructural differences between the alloys are marked and the carbon-containing alloy is characterized by a continuous, high-nickel, high-melting phase which protects the high-magnesium phase present from too-rapid melting, an unpredictable effect.

In preparing alloys of the invention, nickel is melted with carbon to secure solution of carbon in the nickel and the required magnesium addition is then made. Preferably the molten alloy is chill cast, as in metal molds. Desirably, the metal is cast into pig molds of standard size, with precautions to prevent segregation of magnesium during pouring. In this way small metal castings containing an amount of magnesium sufficient to treat a standard weight of molten iron are readily provided and any need for crushing the alloy, with attendant wastage as fines, is avoided. It is surprisingly found that castings made of the carbon-containing alloy of the invention, while being substantially harder than comparable essentially carbonfree alloys, are much tougher. This is a real advantage since breakage of pig castings or other forms is reduced or eliminated and fine material is not generated. The production of castings of substantial size, e.g., cubes at least one inch on an edge, is advantageous since it is found that the tendency to produce smoke and flare is substantially increased with smaller pieces, e.g., pea-size chunks. The castings can be cleaned by tumbling or the like, if necessary, without breakage.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. An addition alloy for introducing magnesium into molten cast iron with minimal accompanying smoke and flare consisting essentially of about 4% to about 5.5% magnesium, about 0.8% to about 1.8% carbon, and the balance essentially nickel, said alloy having a microstructure characterized essentially by the presence of a eutectic structure comprising a nickel-magnesium-carbon compound containing about 12% Mg and 4% carbon in a continuous high nickel phase.

2. An addition alloy in accordance with claim 1 containing about 4.2% to about 4.8% magnesium and about 1.2% to about 1.6% carbon.

3. An addition alloy in accordance with claim 1 having a density at least greater than that of molten cast iron.

References Cited UNITED STATES PATENTS 3,314,787 4/1967 Goodrich et a1 l30 A 2,529,346 11/1950 Millis et al 75-130 A 3,030,205 4/1962 Millis 78130 A WALTER D. SATTERFI-ELD, Primary Examiner US. Cl. X.]R. 

